This invention relates to the field of printed circuit boards, and in particular, relates to the electrodeposition of metal, such as copper, on flat boards.
A typical cell for electroplating comprises a copper cathode and a copper anode, immersed in a solution of copper sulfate (CuSO.sub.4). The cathode comprises the element to be plated. When the electrodes are connected to a source of direct current, the cathode is charged negatively. Some of the electrons from the cathode combine with the positively-charged copper ions (Cu.sup.++) in the solution, to form metallic copper, which is deposited on the surface of the cathode. At the same time, the electric circuit is completed by the discharge of sulfate ions (SO.sub.4.sup.--) on the copper anode, forming copper sulfate which, in turn, dissolves in the solution and restores its original concentration. The reactions at the electrodes can be summarized as follows: EQU Cu.fwdarw.Cu.sup.+2 +2e.sup.- (anode) EQU Cu.sup.+2 +2e.sup.-.fwdarw. Cu (cathode)
Metallic copper is thus transferred from the anode to the cathode. In the example above, the acid in the electrolyte could be sulfuric acid. Other metals and other acids may be used with many variations.
In the electroplating of circuit boards, it is necessary to measure the "throwing power" of the acid in the electrolyte. Throwing power refers to the ability of the acid to penetrate relatively remote areas of the circuit board, such as holes in the board.
There are two principal ways of measuring throwing power which have been known in the prior art. One method is by microsection, that is, the cutting of the board, and observation of the piece through a microscope. The principal disadvantage of this method is that it is very time consuming; the preparation of the board for observation is lengthy and it requires about 8 hours to obtain a measurement. Another method, known in the prior art, for measuring throwing power is the use of the Haring Blum cell, which consists of two or more small panels placed at known distances from an anode. The greater the throwing power, the greater is the amount of metal which is plated on the board further away from the anode. This technique works well for cells using tin/lead, and gold or nickel, but does not work well for acid/copper solutions which are of interest in the present application.
The present invention provides a convenient apparatus for obtaining reliable quantitative estimates of throwing power of an acid.